This invention relates to a plasma processor, which is a semiconductor fabrication apparatus, and, more particularly, to a plasma processor which generates a plasma by electron cyclotron resonance and can effect a uniform plasma process over a large area of a substrate.
FIG. 1 is a sectional, partly schematic view showing the construction of an example of a prior art plasma processor disclosed in Japanese Published Patent Application 57-79621. The plasma processor comprises a plasma generation portion 1 which generates a plasma by electron cyclotron resonance as described in detail later. This plasma generation portion 1 includes a plasma generating vacuum vessel, for example, a glass tube 2, therein. A coil, for example, a solenoid coil 3, arranged around the plasma generation portion 1 is electrically connected with a D.C. power source 4 to generate a magnetostatic field that is non-uniform in the axial direction, namely, z-direction, of the plasma processor. A rectangular waveguide 5 which is directly coupled to the plasma generation portion 1 delivers a microwave electric field in the radial direction, namely, r-direction, of the plasma processor. A magnetron oscillator 7 which is electrically connected with a driving power source 6 comprises microwave generation/supply means together with the aforementioned rectangular waveguide 5. Magnetron oscillator 7 feeds linearly-polarized microwaves to the rectangular waveguide 5. A gas, for example, a reaction gas, is fed to the plasma generating glass tube 2 through a gas supply pipe 8.
The plasma processor comprises a plasma reaction portion 9. In this plasma reaction portion 9, there is disposed a stage 10 on which a substrate 11 to be processed with the plasma is placed. An exhaust pipe 12 for exhausting the used gas is connected to the lower part of the plasma reaction portion 9.
An electron performs a well-known cyclotron motion in a magnetostatic field B, and the angular frequency .omega.c of the cyclotron motion is expressed by .omega..sub.c =eB/m (where e denotes the absolute value of the electronic charge and m denotes the mass of the electron). Letting .omega. denote the angular frequency of the microwave electric field E in the plasma generation portion 1, when the cyclotron resonance condition of .omega.=.omega.c holds, the microwave energy is continuously supplied to the electron, and the energy of the electron increases. FIG. 2 is a diagram showing the resonance region in the radial direction r from the center to the side wall surface of the plasma generation portion 1 and in the axial direction z from the top wall surface to the bottom of the same. A curve from point A to point B is obtained by connecting the points of magnetic field intensities at which the magnetostatic field intensity B.sub.z (z) in the z-direction causes a resonance with the microwave electric field E.
Under such cyclotron resonance conditions, a gas of proper pressure is introduced through the gas supply pipe 8. Then, the electrons generated in a preliminary discharge are continuously supplied with microwave energy, attain a high energy state, and the plasma is developed through the process of collisions between the electrons and gas ions. The microwave energy is further injected into the plasma thus developed under the resonance conditions.
Accordingly, assuming by way of example that the gas introduced through the gas supply pipe 8 is SiH.sub.4, the microwave energy is properly adjusted in conjunction with the pressure of the gas, whereby the species, concentrations, and/or energy levels of respective ions, such as Si.sup.+, SiH.sup.+, SiH.sub.2.sup.+, and SiH.sub.3.sup.+, can be controlled, and simultaneously, the species, concentrations, and/or energy levels of radicals, such as Si.sup.* and SiH.sub.x.sup.* can be controlled.
Meanwhile, an axial force F.sub.z given by the following equation acts on the electrons because of the non-uniform magnetostatic filed B(z) so that the electrons are accelerated in the axial direction: EQU F.sub.z =-.mu.(dB/dz)+(.omega..sub.o /B.sub.o).multidot.((dB/dz)/(1+m/M))
where .mu. denotes the magnetic moment of an electron, B is the magnetic flux density, z is the distance in the axial direction, .omega..sub.o is the frequency of the circular motion of the electron, B.sub.o is the magnetic flux density in the plasma generation portion 1, m is the mass of the electron, and M is the mass of the gas ions.
Accordingly, the electrons in the plasma generated inside the plasma generation portion 1 in FIG. 1 are axially accelerated toward the plasma reaction portion 9. In consequence, an electrostatic field E.sub.o (z) which accelerates the ions is established in the axial direction within the plasma. This electrostatic field E.sub.o (z) accelerates the plasma as a whole in the axial direction, so that a plasma stream 13 extending in the axial direction appears in the plasma reaction portion 9. Since magnetic lines of forces created by the solenoid coil 3 have radial components in the plasma reaction portion 9, the plasma stream 13 diverges and spreads along the magnetic lines of force.
Such a plasma processor can be applied to various surface treatment processes including plasma etching, plasma CVD, and plasma oxidation and can effectively perform these processes.
With the prior art plasma processor utilizing electron cyclotron resonance, the microwaves within the rectangular waveguide are usually linearly-polarized. As an electromagnetic wave propagates in a plasma, however, the clockwise wave component is effective in the generation of the plasma in the processor as stated above. Accordingly, the prior art processor has had the problem that a counterclockwise component of the microwave propagating through the rectangular waveguide does not contribute to the generation of the plasma.
Moreover, in the prior art plasma processor, microwaves are fed through the rectangular waveguide which is directly coupled to the plasma generation portion. Since, however, the cross-sectional area of the plasma generating glass tube is considerably larger than that of the rectangular waveguide, the microwaves entering the plasma generation portion 1 are radiated through an aperture. As a result, the microwave energy density in the radial direction of the plasma generating glass tube is strong at the center and weak near the wall surface. Consequently, the generated plasma is dense at the central part and thin near the wall surface. This variation has led to the problem that a substrate of large diameter is difficult process uniformly.